1. Field of the Invention
The present invention relates to an electromagnetic actuator for controlling the amount of fluid passing therethrough. The actuator of the present invention can be used for controlling the amount of intake air for an internal combustion engine, for example.
2. Description of the Prior Art
A solenoid valve having a valve body which opens and closes an air path based on whether or not the solenoid is energized is incorporated into an electromagnetic actuator for controlling an idling speed of an internal combustion engine. The solenoid coil is energized in accordance with a signal having a duty ratio. The valve body changes its position between an open position and a closed position in a short time in response to that signal so that the air passing through the valve body is substantially continuously controlled.
The conventional type of the electromagnetic actuator, however, has an operational difficulty. An uncontrolled amount of air, the volume of which is larger than a predetermined volume intended to pass when the valve is opened in the normal manner, passes through the valve body when the valve body opens slightly as (shown by dotted line A in FIG. 2). This effect is caused by a repulsive force being generated when the valve body hits the valve seat, i.e. the valve body tends to "bounce" when it closes. The repulsive force causes the valve body to move further from the valve seat than it would move if controlled by the duty ratio alone.
In order to restrain the hitting of the valve body, a soft material such as rubber can be coated on the surface of the valve body and/or on the valve seat. Adding a rubber coating, however, makes the manufacturing process of the valve considerably more complicated and raises the cost of production. Furthermore, the soft material provided between the valve body and the valve seat may cause the valve body to adhere to the valve seat. Accordingly, accurate control of the lift of the valve body cannot be obtained.